High-speed communications, medical applications, and semiconductor processing would all benefit from a laser that can produce stable, high-repetition-rate pulses at 1.55 micrometers. One goal of the Naval Research Laboratory's program in ultrahigh-speed communications research is to develop such an optical source, and an all-fiber laser has been developed that produces pulses from 0.74 to 2 picoseconds in duration at gigahertz repetition rates.
The laser is in effect a polarization-maintaining (PM) ring laser harmonically modelocked by a LiNbO3 modulator, and up to 10,000 pulses circulate in its 200-m-long fiber cavity. The advantage of using PM fiber in the laser is that the instabilities caused by random birefringence variations of non-PM fiber are avoided. The laser also includes a tunable wavelength filter that can control the operating wavelength over its tuning range of approximately 1535-1570 nm.
The laser's pulse duration is determined by the standard Kuizenga-Siegman modelocking equation, which predicts a 5-picosecond duration for a 10-GHz drive frequency. However, if briefer pulses are desired, the laser can operate in a regime in which solitons propagate in the fiber and become compressed in duration. The laser has produced pulses as brief as 740 femtoseconds at a 10-GHz repetition rate in this mode of operation.
Part of the cavity modification that is necessary to produce soliton pulse shaping is to reduce the overall chromatic dispersion in the laser cavity. This is accomplished by adding dispersion-compensating fiber (DCF) to the laser. DCF is not available as PM fiber, and to retain the stable operation of a PM ring laser it is necessary to devise a polarization-maintaining configuration that can incorporate this fiber. By using a fiber-integrated polarizing beamsplitter, Faraday rotator, and mirror, it is possible to construct a birefringence-compensating section of fiber that incorporates non-PM fiber but avoids its instabilities. It is in this section that the DCF is placed. In fact, the arrangement works so well that a standard Er-doped fiber amplifier is also placed in this section of the laser, rather than using a rarer (and more expensive) PM Er-fiber amplifier.
Since the laser is actively modulated, it can be synchronized to repetition rates demanded by other factors in the system. The drive frequency must be an exact multiple of the fundamental cavity mode spacing (f0=c/nL) where c is the speed of light, L is the length of the cavity, and n is the refractive index of the fiber. A simple feedback circuit has been constructed to adjust the length of the cavity by controlling the DC voltage to a piezoelectric cylinder around which is wound a length of fiber. The circuit uses variations in the relative timing of the pulse generated by the laser as an input error signal.
A critical requirement for the laser in communications applications is that it produce an uninterrupted stream of pulses; dropouts in the pulse stream would introduce errors in transmitted data. The laser has a demonstrated dropout rate of less than one in every 1013 pulses. The laser also demonstrates excellent pulse timing and amplitude stability.
The laser has been demonstrated to run for weeks without adjustment, and it is in routine use for experiments in 100-Gb/s communications. The laser could also be frequency-doubled to produce pulses in the range of 775 nm, which could be useful for semiconductor processing or medical applications.
This work was done by Irl Duling, Tom Carruthers, and Michael Dennis of theNaval Research Laboratory. For further information, contact Irl Duling, NRL, Optical Sciences Division, Code 5670, Washington, DC 20375; (202) 767-9351; fax (202) 404-1576. Inquiries concerning rights for the commercial use of this invention should be directed to Dr. Richard Rein, Office of Technology Transfer, Code 1004, NRL, Washington, DC 20375; (202) 767-7230; fax (202) 404-7920.